cmake-generator-expressions(7)

Introduction

Generator expressions are evaluated during build system generation to produce information specific to each build configuration.

Generator expressions are allowed in the context of many target properties, such as LINK_LIBRARIES, INCLUDE_DIRECTORIES, COMPILE_DEFINITIONS and others. They may also be used when using commands to populate those properties, such as target_link_libraries(), target_include_directories(), target_compile_definitions() and others.

This means that they enable conditional linking, conditional definitions used when compiling, and conditional include directories and more. The conditions may be based on the build configuration, target properties, platform information or any other queryable information.

Logical Expressions

Logical expressions are used to create conditional output. The basic expressions are the 0 and 1 expressions. Because other logical expressions evaluate to either 0 or 1, they can be composed to create conditional output:

$<$<CONFIG:Debug>:DEBUG_MODE>

expands to DEBUG_MODE when the Debug configuration is used, and otherwise expands to nothing.

Available logical expressions are:

$<BOOL:...>

1 if the ... is true, else 0

$<AND:?[,?]...>

1 if all ? are 1, else 0

The ? must always be either 0 or 1 in boolean expressions.

$<OR:?[,?]...>

0 if all ? are 0, else 1

$<NOT:?>

0 if ? is 1, else 1

$<IF:?,true-value...,false-value...>

true-value... if ? is 1, false-value... if ? is 0

$<STREQUAL:a,b>

1 if a is STREQUAL b, else 0

$<EQUAL:a,b>

1 if a is EQUAL b in a numeric comparison, else 0

$<IN_LIST:a,b>

1 if a is IN_LIST b, else 0

$<TARGET_EXISTS:tgt>

1 if tgt is an existed target name, else 0.

$<CONFIG:cfg>

1 if config is cfg, else 0. This is a case-insensitive comparison. The mapping in MAP_IMPORTED_CONFIG_<CONFIG> is also considered by this expression when it is evaluated on a property on an IMPORTED target.

$<PLATFORM_ID:comp>

1 if the CMake-id of the platform matches comp, otherwise 0. See also the CMAKE_SYSTEM_NAME variable.

$<C_COMPILER_ID:comp>

1 if the CMake-id of the C compiler matches comp, otherwise 0. See also the CMAKE_<LANG>_COMPILER_ID variable.

$<CXX_COMPILER_ID:comp>

1 if the CMake-id of the CXX compiler matches comp, otherwise 0. See also the CMAKE_<LANG>_COMPILER_ID variable.

$<VERSION_LESS:v1,v2>

1 if v1 is a version less than v2, else 0.

$<VERSION_GREATER:v1,v2>

1 if v1 is a version greater than v2, else 0.

$<VERSION_EQUAL:v1,v2>

1 if v1 is the same version as v2, else 0.

$<VERSION_LESS_EQUAL:v1,v2>

1 if v1 is a version less than or equal to v2, else 0.

$<VERSION_GREATER_EQUAL:v1,v2>

1 if v1 is a version greater than or equal to v2, else 0.

$<C_COMPILER_VERSION:ver>

1 if the version of the C compiler matches ver, otherwise 0. See also the CMAKE_<LANG>_COMPILER_VERSION variable.

$<CXX_COMPILER_VERSION:ver>

1 if the version of the CXX compiler matches ver, otherwise 0. See also the CMAKE_<LANG>_COMPILER_VERSION variable.

$<TARGET_POLICY:pol>

1 if the policy pol was NEW when the ‘head’ target was created, else 0. If the policy was not set, the warning message for the policy will be emitted. This generator expression only works for a subset of policies.

$<COMPILE_FEATURES:feature[,feature]...>

1 if all of the feature features are available for the ‘head’ target, and 0 otherwise. If this expression is used while evaluating the link implementation of a target and if any dependency transitively increases the required C_STANDARD or CXX_STANDARD for the ‘head’ target, an error is reported. See the cmake-compile-features(7) manual for information on compile features and a list of supported compilers.

$<COMPILE_LANGUAGE:lang>

1 when the language used for compilation unit matches lang, otherwise 0. This expression may be used to specify compile options, compile definitions, and include directories for source files of a particular language in a target. For example:

add_executable(myapp main.cpp foo.c bar.cpp zot.cu)
target_compile_options(myapp
  PRIVATE $<$<COMPILE_LANGUAGE:CXX>:-fno-exceptions>
)
target_compile_definitions(myapp
  PRIVATE $<$<COMPILE_LANGUAGE:CXX>:COMPILING_CXX>
          $<$<COMPILE_LANGUAGE:CUDA>:COMPILING_CUDA>
)
target_include_directories(myapp
  PRIVATE $<$<COMPILE_LANGUAGE:CXX>:/opt/foo/cxx_headers>
)

This specifies the use of the -fno-exceptions compile option, COMPILING_CXX compile definition, and cxx_headers include directory for C++ only (compiler id checks elided). It also specifies a COMPILING_CUDA compile definition for CUDA.

Note that with Visual Studio Generators and Xcode there is no way to represent target-wide compile definitions or include directories separately for C and CXX languages. Also, with Visual Studio Generators there is no way to represent target-wide flags separately for C and CXX languages. Under these generators, expressions for both C and C++ sources will be evaluated using CXX if there are any C++ sources and otherwise using C. A workaround is to create separate libraries for each source file language instead:

add_library(myapp_c foo.c)
add_library(myapp_cxx bar.cpp)
target_compile_options(myapp_cxx PUBLIC -fno-exceptions)
add_executable(myapp main.cpp)
target_link_libraries(myapp myapp_c myapp_cxx)

Informational Expressions

These expressions expand to some information. The information may be used directly, eg:

include_directories(/usr/include/$<CXX_COMPILER_ID>/)

expands to /usr/include/GNU/ or /usr/include/Clang/ etc, depending on the Id of the compiler.

These expressions may also may be combined with logical expressions:

$<$<VERSION_LESS:$<CXX_COMPILER_VERSION>,4.2.0>:OLD_COMPILER>

expands to OLD_COMPILER if the CMAKE_CXX_COMPILER_VERSION is less than 4.2.0.

Available informational expressions are:

$<CONFIGURATION>

Configuration name. Deprecated. Use CONFIG instead.

$<CONFIG>

Configuration name

$<PLATFORM_ID>

The CMake-id of the platform. See also the CMAKE_SYSTEM_NAME variable.

$<C_COMPILER_ID>

The CMake-id of the C compiler used. See also the CMAKE_<LANG>_COMPILER_ID variable.

$<CXX_COMPILER_ID>

The CMake-id of the CXX compiler used. See also the CMAKE_<LANG>_COMPILER_ID variable.

$<C_COMPILER_VERSION>

The version of the C compiler used. See also the CMAKE_<LANG>_COMPILER_VERSION variable.

$<CXX_COMPILER_VERSION>

The version of the CXX compiler used. See also the CMAKE_<LANG>_COMPILER_VERSION variable.

$<TARGET_FILE:tgt>

Full path to main file (.exe, .so.1.2, .a) where tgt is the name of a target.

$<TARGET_FILE_NAME:tgt>

Name of main file (.exe, .so.1.2, .a).

$<TARGET_FILE_DIR:tgt>

Directory of main file (.exe, .so.1.2, .a).

$<TARGET_LINKER_FILE:tgt>

File used to link (.a, .lib, .so) where tgt is the name of a target.

$<TARGET_LINKER_FILE_NAME:tgt>

Name of file used to link (.a, .lib, .so).

$<TARGET_LINKER_FILE_DIR:tgt>

Directory of file used to link (.a, .lib, .so).

$<TARGET_SONAME_FILE:tgt>

File with soname (.so.3) where tgt is the name of a target.

$<TARGET_SONAME_FILE_NAME:tgt>

Name of file with soname (.so.3).

$<TARGET_SONAME_FILE_DIR:tgt>

Directory of with soname (.so.3).

$<TARGET_PDB_FILE:tgt>

Full path to the linker generated program database file (.pdb) where tgt is the name of a target.

See also the PDB_NAME and PDB_OUTPUT_DIRECTORY target properties and their configuration specific variants PDB_NAME_<CONFIG> and PDB_OUTPUT_DIRECTORY_<CONFIG>.

$<TARGET_PDB_FILE_NAME:tgt>

Name of the linker generated program database file (.pdb).

$<TARGET_PDB_FILE_DIR:tgt>

Directory of the linker generated program database file (.pdb).

$<TARGET_BUNDLE_DIR:tgt>

Full path to the bundle directory (my.app, my.framework, or my.bundle) where tgt is the name of a target.

$<TARGET_BUNDLE_CONTENT_DIR:tgt>

Full path to the bundle content directory where tgt is the name of a target. For the macOS SDK it leads to my.app/Contents, my.framework, or my.bundle/Contents. For all other SDKs (e.g. iOS) it leads to my.app, my.framework, or my.bundle due to the flat bundle structure.

$<TARGET_PROPERTY:tgt,prop>

Value of the property prop on the target tgt.

Note that tgt is not added as a dependency of the target this expression is evaluated on.

$<TARGET_PROPERTY:prop>

Value of the property prop on the target on which the generator expression is evaluated.

$<INSTALL_PREFIX>

Content of the install prefix when the target is exported via install(EXPORT) and empty otherwise.

$<COMPILE_LANGUAGE>

The compile language of source files when evaluating compile options. See the unary version for notes about portability of this generator expression.

Output Expressions

These expressions generate output, in some cases depending on an input. These expressions may be combined with other expressions for information or logical comparison:

-I$<JOIN:$<TARGET_PROPERTY:INCLUDE_DIRECTORIES>, -I>

generates a string of the entries in the INCLUDE_DIRECTORIES target property with each entry preceded by -I. Note that a more-complete use in this situation would require first checking if the INCLUDE_DIRECTORIES property is non-empty:

$<$<BOOL:${prop}>:-I$<JOIN:${prop}, -I>>

where ${prop} refers to a helper variable:

set(prop "$<TARGET_PROPERTY:INCLUDE_DIRECTORIES>")

Available output expressions are:

$<0:...>

Empty string (ignores ...)

$<1:...>

Content of ...

$<JOIN:list,...>

Joins the list with the content of ...

$<ANGLE-R>

A literal >. Used to compare strings which contain a > for example.

$<COMMA>

A literal ,. Used to compare strings which contain a , for example.

$<SEMICOLON>

A literal ;. Used to prevent list expansion on an argument with ;.

$<TARGET_NAME:...>

Marks ... as being the name of a target. This is required if exporting targets to multiple dependent export sets. The ... must be a literal name of a target- it may not contain generator expressions.

$<TARGET_NAME_IF_EXISTS:...>

Expands to the ... if the given target exists, an empty string otherwise.

$<LINK_ONLY:...>

Content of ... except when evaluated in a link interface while propagating Transitive Usage Requirements, in which case it is the empty string. Intended for use only in an INTERFACE_LINK_LIBRARIES target property, perhaps via the target_link_libraries() command, to specify private link dependencies without other usage requirements.

$<INSTALL_INTERFACE:...>

Content of ... when the property is exported using install(EXPORT), and empty otherwise.

$<BUILD_INTERFACE:...>

Content of ... when the property is exported using export(), or when the target is used by another target in the same buildsystem. Expands to the empty string otherwise.

$<LOWER_CASE:...>

Content of ... converted to lower case.

$<UPPER_CASE:...>

Content of ... converted to upper case.

$<MAKE_C_IDENTIFIER:...>

Content of ... converted to a C identifier. The conversion follows the same behavior as string(MAKE_C_IDENTIFIER).

$<TARGET_OBJECTS:objLib>

List of objects resulting from build of objLib. objLib must be an object of type OBJECT_LIBRARY.

$<SHELL_PATH:...>

Content of ... converted to shell path style. For example, slashes are converted to backslashes in Windows shells and drive letters are converted to posix paths in MSYS shells. The ... must be an absolute path.

$<GENEX_EVAL:...>

Content of ... evaluated as a generator expression in the current context. This enables consumption of generator expressions whose evaluation results itself in generator expressions.

$<TARGET_GENEX_EVAL:tgt,...>

Content of ... evaluated as a generator expression in the context of tgt target. This enables consumption of custom target properties that themselves contain generator expressions.

Having the capability to evaluate generator expressions is very useful when you want to manage custom properties supporting generator expressions. For example:

add_library(foo ...)

set_property(TARGET foo PROPERTY
  CUSTOM_KEYS $<$<CONFIG:DEBUG>:FOO_EXTRA_THINGS>
)

add_custom_target(printFooKeys
  COMMAND ${CMAKE_COMMAND} -E echo $<TARGET_PROPERTY:foo,CUSTOM_KEYS>
)

This naive implementation of the printFooKeys custom command is wrong because CUSTOM_KEYS target property is not evaluated and the content is passed as is (i.e. $<$<CONFIG:DEBUG>:FOO_EXTRA_THINGS>).

To have the expected result (i.e. FOO_EXTRA_THINGS if config is Debug), it is required to evaluate the output of $<TARGET_PROPERTY:foo,CUSTOM_KEYS>:

add_custom_target(printFooKeys
  COMMAND ${CMAKE_COMMAND} -E
    echo $<TARGET_GENEX_EVAL:foo,$<TARGET_PROPERTY:foo,CUSTOM_KEYS>>
)